Field of the Invention
The present invention relates to a method for estimating the moisture content of fabrics in a drier during the drying cycle and for enabling an automatic cycle termination once a desired moisture level is reached.
As a drier it is here meant any appliance for drying clothes and fabrics and which is preferably for domestic use. The drier can be either a condensing drier or an air-vented drier. Preferably it is a condensing tumble drier for domestic use. The invention support either the heating system based on “electrical resistance” or systems known as “heat pump” system that makes use of a refrigerant cycle to heat the air.
Description of the Related Art
Methods for terminating the drying cycle of a drier when the desired drying level is achieved are known in the art.
A known simple method is based on the drying time length. The level of the residual moisture is assumed to be directly correlated with the drying time. The control unit counts the drying time and stops the drying cycle after the time manually set by the user has elapsed, leaving a certain level of residual moisture in the fabrics. This method requires the user to estimate the drying time and to set manually the dryer accordingly. As a result, this method does not provide excellent performances but drives to a waste of time and energy.
Another known method uses conductibility sensors in the form of metal stripes placed either in the drum or on the drum lifters. With such sensors it is measured the instantaneous resistance of the clothes contacting the stripes, which is linked with their moisture content: the higher is the resistance the dried are the clothes. The conductibility value read through these sensors is compared to a predefined/selected threshold value stored in the memory of the control unit. In the memory are stored a certain numbers of threshold values, each of them corresponding to a certain dryness level according to the number of drying programs offered by the machine to the user.
This method requires a quite complex manufacturing operation for fixing the conductibility sensors in the drying chamber. This method is particularly efficient when implemented in tumble driers, but it is less efficient when applied in driers wherein fabrics are not agitated/moved during the drying process. The method is however unsatisfactory when drying bulky items, that is items presenting high volume/surface and low mass. In these cases in fact, conductibility sensors do no enter in contact with the interior portion of the items.
Moreover, from EP-A-2034086 filed by the same applicant, the moisture retention of fabric inside the dryer is estimated using two temperature sensors: a first sensor placed nearby the heater at the inlet of the drying chamber, and used to avoid over temperatures; and a second sensor placed the near or at the outlet of the drying chamber used to control the air temperature linked with the clothes. In FIG. 1 it is possible to see the positioning of the two temperature sensors 1, 2.
The air temperature of the drying chamber is controlled with a closed loop temperature control, as described on FIG. 2 by modulating/controlling the power to be supplied by the heater. The moisture estimation module uses in input the heater temperature signal and the drying chamber temperature signal, measured through the two temperature sensors. When the estimated moisture retention matches or decreases below the desired/set value, the drying cycle is stopped. This method is preferably applicable to a tumble drier.
However, more than one temperature sensor is required to estimate the moisture content.
Further known methods use a relative humidity sensor or a weight sensor, and stop the drying cycle when a predefined threshold limit is reached. These methods require an “ad hoc” expensive sensor, while the other described methods use the same temperature sensors used for controlling the heater temperature.
Finally, in the art are also known methods using a single temperature sensor to automatically estimate the mass amount of clothes within the dryer. With this information the duration of the drying cycle is established accordingly. To be reliable this method requires having in input also the type of fabrics to be dried. This information is normally not accurate if automatically calculated, and it is preferably required to be inputted by the user, causing to the process become a manual process.